The turbine blades of industrial gas turbines and aircraft engines operate in a high temperature environment, where the temperatures regularly reach between 600° C. and 1500° C. Moreover, the general trend is to increase the turbine operating temperatures to increase output and engine efficiencies. Thermal stresses placed on the turbine blades associated with these conditions are severe.
In general, turbine blades undergo high level of mechanical stress due to the forces applied via the rotational speed of the turbine. These stresses have been driven to even higher levels in an effort to accommodate turbine blade design that include higher annulus areas that yield higher output torque during operation. In addition, the desire to design turbine blade tip shrouds of greater surface area has added addition weight to the end of the turbine blade, which has further increased the mechanical stresses applied to the blade during operation. When these mechanical stresses are coupled with the severe thermal stresses, the result is that turbine blades operate at or close to the design limits of the material. Under such conditions, turbine blades generally undergo a slow deformation, which is often referred to as “metal creep.” Metal creep refers to a condition wherein a metal part slowly changes shape from prolonged exposure to stress and high temperatures. Turbine blades may deform in the radial or axial direction.
As a result, the turbine blade failure mode of primary concern in the aft end stages of a gas turbine is metal creep, and particularly radial metal creep (i.e., elongation of the turbine blade). If left unattended, metal creep eventual may cause the turbine blade to rupture, which may cause extreme damage to the turbine unit and lead to significant repair downtime. In general, conventional methods for monitoring metal creep in turbine blades include either: (1) attempting to predict the accumulated creep elongation of turbine blades as a function of time through the use of analytical tools such as finite element analysis programs, which calculate the creep strain from algorithms based on creep strain tests conducted in a laboratory on isothermal creep test bars; or (2) visual inspections and/or hand measurements conducted during the downtime of the unit. However, the predictive analytical tools often are yield inaccurate. And, the visual inspections and/or hand measurements are labor intensive, costly, and, often, also yield inaccurate results.
In any case, inaccurate predictions as to the health of the turbine blade, whether made by using analytical tools, visual inspection or hand measurements, may be costly. On the one hand, inaccurate predictions may allow the turbine blades to operate beyond their useful operating life and lead to a turbine blade failure, which may cause severe damage to the turbine unit and repair downtime. On the other hand, inaccurate predictions may decommission a turbine blade to early (i.e., before its useful operating life is complete), which results in inefficiency. Accordingly, the ability to accurately monitor the metal creep displacement of turbine blades may increase the overall efficiency of the turbine engine unit. Such monitoring may maximize the service life of a turbine blades while avoiding the risk of turbine blade failure. In addition, if such monitoring could be done without the expense of time-consuming and labor-intensive visual inspections or hand measurements, further efficiencies would be realized. Thus, there is a need for improved systems for monitoring or measuring the metal creep displacement of turbine blades.